The state of art knows single action semiautomatic pistols such as Colt 1911 gun, whose hammer can be cocked by the thumb or by sliding back the breechblock either manually or due to a shot. In both cases, the hammer always stands in the same position, somewhat backwards, it being necessary only some pressure of the forefinger on the trigger to fire off.
Guns of this kind can be locked with the hammer in the cocked position. The only way to decock the gun after its unlocking is to thumb the hammer and lead it down, little by little, after pull the trigger.
The risk of accidental shots due to the trigger sensitivity when the gun is unlocked requires its carrying inside a holster always locked or with the hammer down and unlocked. In both cases, a sudden draw out of the pistol requires a conscious act to either unlock or to cock it with the thumb.
By this reason, double action pistols were developed. These, when carried unlocked and in perfect safety conditions, can fire the first shot just by pulling the trigger which, in along stroke, such as with revolvers, cocks the hammer, releasing it further for the shooting in a continuous motion. After the first shot, the hammer stands cocked like with single action pistols.
Nowadays, there are four main kinds of safety devices for double action pistols, among which there are some which afford or help afford a safe hammer decocking. The oldest one is safety lever assembled in the sliding breechblock, which nowadays appears in WHALTER mod. P-38, PPK, PP and SMITH & WESSON cal. 9 mm and .45 pistol's., further to RUCER mod, P-85 and BERETA mod. 92. In all these models, when the latch bolt is turned down to uncock the hammer, it remains down positioned, requiring an obligatory and conscious turning up for double action shooting, Exception is made with regard to the STEIR GB 9 mm pistol whose latch, when lowered to uncock the hammer safely turns back automatically to the unlock position to allow immediate double action.
Similar device can be found in pistols such as HECKLER & KOCH P9 and STAR M30, which have a safety lever assembled in the sliding breechblock, When activated, it blocks the firing pin but does not decock the hammer, which requires further pulling of the trigger. This devices requires the pulling of the trigger, after duly setting the locking lever to the lock position, in order to decock the hammer without shooting, it being necessary the unlocking of the gun for further double action shooting.
Such locking devices have yet the inconvenience of being assembled in the sliding breechblock, making difficult the operation with the thumb, and making possible an unintentional action, when operating manually over the grip to hammer cocking, to tuck the pistol in a narrow holster or pull it out.
Another device is the decocking lever, appearing in the SIG-SAUER P-226 pistols, which is not properly a safety device, but a lever located in the front grip, which, when lowered with the trigger pulled it safely decocks the hammer. After that, the lever turns up automatically to the initial position, allowing double action shooting.
Another device employs a lock assembled into the frame that just blocks the sear, and requires that the hammer be lowered through manual control, while pulling the trigger for decocking, This device appears in the BERETA 92 S, BENELLI, BERNARDELLI, CZ-75 e TAURUS PT 99 e 92, among others.
This device allows to carrying above pistols cocked and locked, in the way some self defense experts recommend based on the COLT 1911, but the double action advantage is jeopardized when the gun is set to operate in this condition due to the dangerous decocking required.